Transponders, such as RFID or NFC systems, rely on electromagnetic waves exchange and especially in the near-field on inductive coupling with a reader for power and data transmission. The transponder often comprises an electronic data carrying device, usually a single microchip and an antenna, e.g., a large area coil that functions as an antenna.
Data may be transferred from the transponder to the reader using load modulation. If a transponder is placed within the magnetic alternating field of the readers antenna, then this draws energy from the electromagnetic field. This additional power consumption can be measured at the reader. By switching a load resistance at the transponders antenna, data may be transferred through this effect.
However, as transponders are getting smaller, the load modulation effect becomes less effective. A small transponder antenna does not accumulate enough energy from the reader field. Therefore the coil voltage for demodulation is too low even on the transistor used for load modulation. Also the coupling back to the reader is poor.
The data transfer may be improved by adding a battery power supply. In this case active modulation of a carrier wave may be used instead of passive load modulation. This approach has the drawback that the carrier frequency (fc) used for the modulation needs to have exactly the same phase as the carrier frequency that is received by the transponder. Such synchronization is difficult to achieve and requires additional synchronization circuits.